Process for producing potassium azide



United States Patent 3,455,648 PROCESS FOR PRODUCING POTASSIUM AZIDE Richard Shaw and James K. Wood, St. Hilaire, Quebec,

Canada, assignors to Canadian Industries Limited, Montreal, Quebec, Canada No Drawing. Filed May 31, 1966, Ser. No. 553,673 Claims priority, application Canada, June 12, 1965, 933,162 Int. Cl. C01b 21/08 US. Cl. 23-101 1 Claim ABSTRACT OF THE DISCLOSURE Substantially pure potassium azide in crystal form is obtained by subjecting to crystallization conditions an aqueous solution of sodium azide and potassium hydroxide in which the mole ratio of potassium to sodium is about two to one.

This invention relates to potassium azide and in particular to a process for the manufacture of potassium azide which is free from the hazards and other disadvantages of processes known heretofore.

Employing traditional procedures, potassium azide may be prepared, for example, by the Wislicenus method wherein metallic potassium is heated until molten and dry ammonia is bubbled through the molten metal to form potassium amide with the liberation of hydrogen gas. The potassium amide so formed is then reacted with nitrous oxide to yield an equimolecular mixture of potassium hydroxide and potassium azide. The potassium azide portion may then be fractionally crystallized from water. This method for producing potassium azide is not attractive commercially because of the high cost and limited availability of potassium metal. Additionally, because of the extreme reactivity of potassium metal with water, special precautions are necessary to exclude all moisture in processes where potassium metal is used.

Another known textbook method of producing potassium azide is the preparation, firstly, of hydrogen azide (hydrazoic acid) from sodium azide and, secondly, the conversion into potassium azide of the hydrogen azide. Because hydrogen azide is an unstable and explosive compound, it must be diluted with water, or in the vapour phase with a carrier gas such as water vapour, in order to prevent detonation. In addition, because of its volatility, hydrogen azide presents a significant toxicity hazard. These characteristics of hydrogen azide make such a process unattractive since special and costly handling re-' quirements are needed to insure safe operation.

Yet another known method of obtaining potassium azide is by the interaction of butyl nitrite and hydrazine in alcoholic potassium ethylate or potassium hydroxide. Hydrazine, however, is an expensive reagent which is only difiicultly manufactured. This process for the production of potassium azide is, therefore, normally unattraclive on a commercial scale because of the high costs involved.

All of the aforementioned known methods for producing potassium azide suffer from one or more disadvantages which make these processes impracticable from a commercial viewpoint since the processes are either excessively costly or unduly hazardous or both. In addition, the yields or the purity of the potassium azide produced may be disappointing and additional purification process steps may be required in order to obtain a satisfactory product.

It is the primary object of this invention to provide a process for the production of potassium azide which overcomes all the atorementioned disadvantages of prior art I CC.

processes. Other objects of the invention will appear hereinafter.

Broadly speaking, the process of this invention comprises subjecting to crystallization conditions an aqueous solution of sodium azide and potassium hydroxide in which the mole ratio of potassium to sodium is about two to one, whereby nearly pure potassium azide is obtained in crystal form.

The details of the process are simple since usual chemical processing equipment may be utilized and the reactants employed are readily available and relatively inexpensive. The sodium azide used may, for example, be of the type manufactured by the Wislicenus method, being normally more than 99% pure. The potassium hydroxide or commercial caustic potash used is normally readily available.

In the process of the invention, about two moles of potassium as potassium hydroxide and one mole of sodium as sodium azide are dissolved with stirring in a quantity of hot water. Upon evaporation of the water or upon simple cooling of the solution or a combination of these steps, a precipitate of nearly pure potassium azide crystals is obtained.

Although, in the process of the invention, the mole ratio of potassium hydroxide to sodium azide may be varied over a relatively wide range, it has been found that a ratio in excess of about two moles of potassium to one mole of sodium results in wasted material because of a surplus of potassium in the reaction medium. A ratio of less than 2 moles of potassium to one mole of sodium results in a smaller yield of substantially pure potassium azide in a single crystallization.

The discovery that when a given mole ratio of sodium azide and potassium hydroxide are dissolved in a water solution and the solution then treated so as to allow a precipitation of the dissolved salt, nearly pure potassium azide is produced is surprising. It would normally be expected by one skilled in the art that substantially pure potassium azide could only be obtained from such a solution, if at all, under specific phase conditions of temperature and pressure. It would similarly be expected by one skilled in the art that a precipitate obtained from such a solution under simple conditions of cooling or concentration or both would be comprised of mixtures of the reaction products, namely crystals of both potassium azide and sodium azide.

The following example illustrates the process of the invention but the latter is in no manner to be limited in scope to the embodiments disclosed.

EXAMPLE Nearly pure potassium azide was produced employing the following process steps:

(a) Three hundred and fifty-two pounds of 99% pure sodium azide and 608 pounds of potassium hydroxide flakes were dissolved with stirring in 2240 pounds of water. The potassium hydroxide was 91% pure, the main impurities being potassium carbonate (less than 2%), potassium chloride (less than 1.5%) and moisture.

('b) The solution was filtered through an inert filter cloth to remove any insoluble matter present and placed in a vacuum evaporator.

(c) Water was evaporated from the solution (using 50 p.s.i.g. steam heat and a vacuum of about 20 inches of mercury) until potassium azide crystals had formed. The specific gravity of the mother liquor was 1.34 at 60 F.

(d) The mother liquor and crystals were cooled to 32 C. and passed into a basket centrifuge where the collected crystals were washed with about pounds of water.

(e) The crystals were collected and dried in trays at 110 C. for 20 hours. The weight of the dried product crystals was 180 pounds.

(f) The mother liquor with added wash water from the basket centrifuge was placed into a second vacuum evaporator and water was evaporated until a second crop of crystals was obtained. The specific gravity of the mother liquor was 1.50 at 60 F.

(g) The mother liquor and crystals were cooled to 32 C. and passed into a basket centrifuge where the second crop crystals were collected. The mother liquor was sent to a waste azide destruction unit.

(h) The unwashed, undried second crop crystals weighing 260 pounds contained sodium and potassium azide and assayed about 85% azide as potassium azide. The second crop crystals were dissolved in 800 pounds of hot Water and slaked lime was added and the mix heated for 6 hours at 90 C. to precipitate any soluble carbonate present.

(i) The carbonate free solution from step (11) was mixed with a quantity of solution as prepared in step (a), filtered and transferred to a vacuum evaporator. Process steps (c) to (i) were repeated. The first crop product obtained in step (e) was 380 pounds of potassium azide.

The collected, dried, first crop product analyzed from 2 97.1% to 98.1% potassium azide by the Pickerring Laboratory method of analysis of azide with ammonium hexanitratoeerate reagent (method of Inspection Board of Canada) and from 98.5% to 102.2% potassium azide by the sodium tetraphenyl boron method for potassium (U.S. National Plant Food Institute Analytical Method 2.410). Hydroxyl ion was about 0.03% and carbonate ion about 0.15%.

What we claim is:

1. A process for preparing potassium azide which comprises dissolving together in an aqueous medium approximately two mole proportions of potassium as potassium hydroxide and one mole proportion of sodium as sodium azide, evaporating the water in order to crystallize potassium azide, washing said potassium azide crystals with water and drying.

References Cited UNITED STATES PATENTS 1,353,805 9/1920 Smelling 23-101 FOREIGN PATENTS 129,152 7/ 1919 Great Britain.

OTHER REFERENCES Mellor: Comprehensive Treat. on Inorg. and Theor. Chem, vol. 8, p. 347 (1928).

OSCAR R. VERTIZ, Primary Examiner G. 0. PETERS, Assistant Examiner 

